Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. Mechanical intervention via stents has reduced the rate of restenosis; restenosis, however, is still a significant clinical problem. Accordingly, stents have been modified to perform not only as a mechanical scaffolding, but also to provide biological therapy.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
A common method of medicating a stent is by depositing a polymeric coating, impregnated with the therapeutic substance, on the surface of the stent. A polymer dissolved in a solvent is applied to the stent. A therapeutic substance can be dissolved or dispersed in the composition. The solvent is allowed to evaporate to form the coating. The application of the composition can be performed by spraying the composition on the stent or immersing the stent in the composition.
The solvents employed with the composition can be categorized as having a high vapor pressure or low vapor pressure. Non-volatile solvents evaporate very slowly from the composition causing coating defects such as inconsistency in the coating thickness and formation of “cob webs” or “pool webs” between the stent struts. A solution to this problem is to coat the stent at elevated temperatures to increase the evaporation rate of the solvent. However, not all drugs are stable at elevated temperatures. Volatile solvents have the tendency to evaporate very quickly from the composition resulting in a coating which has a powdered consistency and adheres poorly to the surface of the stent. Accordingly, what is needed is an apparatus and process for coating stents that does not suffer from the aforementioned drawbacks.